FLAVOCYTOCHROME-B2 FROM BAKERS-YEAST - COMPUTER-SIMULATION STUDIES OF A NEW KINETIC SCHEME FOR INTRAMOLECULAR ELECTRON-TRANSFER

  • 1 January 1980
    • journal article
    • research article
    • Vol. 106  (1) , 151-159
Abstract
The use of L-(+)-[2-2H]lactate as substrate instead of unlabeled L-(+)-lactate induces a lowering of the flavin reduction rate of cytochrome b2 by a factor of 8 [D. Pompon, M. Iwatsubo and F. Lederer (1980)]. This high isotope effect enabled the study of the electron transfer between prosthetic groups at a very low rate of electron entry. The kinetic scheme for electron transfer in cytochrome b2 proposed by Capeillere-Blandin is examined here in the light of the kinetic data previously reported. This study indicates some disagreements, particularly at a low rate of electron entry. New kinetic schemes capable of explaining data obtained with deuterated lactate are proposed. These new schemes differ from that of Capeillere-Blandin in that: the hypothesis of simultaneous prosthetic group reduction for the 2 protomers of 1 given dimer is abandoned; the limiting step of the slow phases of heme and flavin reduction is a slow interprotomer electron exchange between a heme pair, a flavin pair or heme and flavin; a rather fast conformational change controlled by the redox state of heme or flavin of one protomer can modulate the rate of electron transfer in another protomer. These new kinetic schemes allow the determination of the rate of intraprotomer and interprotomer electron transfer and to decide precisely how these steps are modified by the proteolytic cleavage of intact enzyme to cleaved enzyme.